Australian Field Ornithology 2012, 29, 169–181 Contributions to the reproductive effort in a group of plural-breeding Pied Butcherbirds Cracticus nigrogularis D.G. Gosper 39 Azure Avenue, Balnarring VIC 3926, Australia Email: [email protected] Summary. Concurrent nesting by two females from a single social group of Pied Butcherbirds Cracticus nigrogularis is described. Young fledged from two nests in the first 2 years, but breeding failed in the following two seasons after Australian Magpies C. tibicen displaced the group from its original nest-tree. The two breeding females in the Pied Butcherbird group constructed their nests and incubated synchronously in the same tree without conflict. Only the females constructed the nest and incubated the eggs. Other group members fed the females before laying and during incubation. Females begged for food using a display resembling that of juvenile Butcherbirds. Most (and probably all) members of the group fed the nestlings and fledglings, with multiple members delivering food to the young in both nests and removing or eating faecal sacs. Immatures at the beginning of their second year, with no previous experience, performed the full range of helping tasks, including the provisioning of females from the pre-laying stage. During the nestling stage, the group fed on nectar from Silky Oaks Grevillea robusta, but did not feed this to the young. Introduction The Pied Butcherbird Cracticus nigrogularis (Artamidae) may breed co-operatively (Rowley 1976; Dow 1980; Clarke 1995). Higgins et al. (2006) considered it to be an occasional co-operative breeder, although the only detailed study to date (i.e. Robinson 1994) suggested that, despite not being an obligate co-operative breeder, the species usually breeds in territorial co-operative groups. Most studies on Pied Butcherbirds mention ‘helpers’ (individuals other than the breeding female and male) feeding the nestlings and fledglings (Thomas 1951; Courtney & Marchant 1971; Higgins et al. 2006), behaviours that have been photographed by C.A. Webster (see Boles 1995; Debus 1996). Co-operative breeders, including some species in the Artamidae, vary greatly in their social organisation, mating systems, and extent of co-operation (Hughes & Mather 1991; Hughes et al. 1996; du Plessis & Emlin 1999; Rowley 1999; Russell & Rowley 2009). One variant is plural breeding, in which more than one female in the group lays eggs, either in the same nest or with each female building her own nest (Curry 1988). Robinson (1994), in a 3-year study of helpers at the nest in 11 Pied Butcherbird territories in farmland in south-eastern Queensland, recorded plural breeding in four groups in which a second (beta) female built a separate nest and produced eggs and sometimes young. Fleay (1953) and Nielsen (1965), also in south-eastern Queensland, referred to what was probably plural breeding. 170 Australian Field Ornithology D.G. Gosper Fleay (1953) described two females simultaneously constructing separate nests in trees ~30 m apart. A single male associated with both females during construction but fed only one. Young fledged from both nests. Nielsen (1965) described a nest containing seven eggs, probably laid by more than one female (based on patterning of the eggs), which was attended by four adults, two of which incubated. In this paper, I describe the breeding activities of a social group of Pied Butcherbirds over consecutive years. The Butcherbirds were found attending two active nests in the same tree, suggesting plural breeding. Behaviour of the territorial group at and near the nest(s) was monitored over four breeding seasons (1994–97) to elucidate the roles and contributions of individual members in the group’s reproductive effort. The territory was again monitored in 2007 to see what changes, if any, had occurred over 10 years. Study area and methods The study area was at Goolmangar (28°42′S, 153°13′E), near Lismore in north-eastern New South Wales. The Pied Butcherbird breeding territory was on the floodplain of Goolmangar Creek, among open pasture used for grazing cattle. The area was flat, mostly treeless, with fence-lines ~300 m apart. The creek was lined by remnant gallery dry rainforest, dominated by introduced Camphor Laurel Cinnamomum camphora, and featuring scattered emergent Forest Red Gums Eucalyptus tereticornis, River She-oaks Casuarina cunninghamiana and Silky Oaks Grevillea robusta. It ran north–south through the territory, which lay primarily on its eastern side. Scattered trees, mainly Forest Red Gums, Hoop Pines Araucaria cunninghamii and Swamp Turpentines Syncarpia suaveolens 300–350 m north, east and south of the nest-sites were rarely visited during nesting. Monitoring of nests In 1994, the nests contained young when discovered in early October. In subsequent years, monitoring began in July, and continued weekly until nest-building began. Thereafter, nests were monitored every 1–4 days (occasionally twice daily) until fledging in early November. A total of 72.2 h of observation was made over four seasons (range 8.4–35.4 h). Nest-watch sessions averaged 1 h (range 0.25–2.2 h). Watches were conducted from 51 minutes before sunrise until 33 minutes after sunset, but most fell in the periods 0530–0930 h and 1630– 1815 h. In 2007, ten 1-hour sessions were logged during late incubation and nestling stages. Nest-watches were made from below a tree 60–70 m from the nest-tree, with good surrounding vision allowing birds away from the nest to be followed. I recorded time of day, identity (if possible) of individual(s) arriving at the nests, the response of the occupant, the activity which followed (e.g. bird added nest material, fed nestlings, passed food to brooder, settled to brood, removed faecal sacs), and time of departure from the nest. Individual Butcherbirds were tracked for up to 35 minutes, during which their direction and/or flight- path to and from the nest(s) were recorded, as was their subsequent destination. Other behaviours at or near the nest-tree, such as nest-defence and foraging, were also recorded. Results All data refer to the period 1994–97 unless otherwise indicated. The Pied Butcherbird group bred, or attempted to breed, in each of the four consecutive breeding seasons (Table 1). The territory boundaries remained unchanged, and no intraspecific confrontations were observed, although Plural breeding in Pied Butcherbirds 171 Table 1. Overview of breeding by a social group of Pied Butcherbirds at Goolmangar, NSW, in 1994–97 and 2007. Year Social group Nests Breeding success Comment 1994 3 adults + 2 (plural) 3 young fledged 2 immatures (helpers) 1995 3 adults + 2 (plural) 3 young fledged 2 adults (helpers) 1996 3 adults + 2 (plural) 2 breeding Butcherbirds 2 immatures 2 (plural) attempts; all nests displaced from (helpers) failed/abandoned original nest-tree before breeding by nesting Magpies 1997 2 adults + 1 Nest failed/ Nesting Magpies 1 near-adult- abandoned again occupied plumaged, original nest-tree 1 immature (helpers) 2007 2 adults 1 2 young fledged Original nest-tree collapsed; Magpies nesting nearby Butcherbirds in a neighbouring territory were heard calling. In 2007, the nest was in a tree 100 m farther north-east, and the breeding pair clashed with other Butcherbirds inside the southern edge of the original territory. Locations of nests In 1994 and 1995, the group nested in an isolated senescent River She-oak, in a depression 60 m from the edge of the riparian zone. Both nests (A and B) were in forks 3.8 m apart, on opposite sides of the trunk. Nest A was 6.5 m and Nest B 8.3 m above ground, the latter being at approximately two-thirds the height of the tree. The Butcherbirds used three dead trees 60 m south-west, standing well above the adjacent riparian zone canopy, as perches for singing, preening and vigilance. In 1996, Australian Magpies Cracticus tibicen nested in the original nest-tree before the Butcherbirds started breeding, thus displacing the Butcherbirds, which subsequently built two nests (X1 and Y1) in adjoining she-oaks on the eastern bank of the creek, 70 m north-west of the original nest-tree. These nests were abandoned, as were two further nests (X2 and Y2) in non-adjacent she-oaks on the opposite bank. In 1997 Magpies re-occupied the original nest-tree, and the Butcherbirds built a single nest low in the isolated Hoop Pine 160 m south-east of their original nest-tree. By 2007, the original nest-tree had collapsed, and the Butcherbirds built a nest 100 m north-east of it in an isolated Rough-leaved Elm Aphananthe philippinensis 172 Australian Field Ornithology D.G. Gosper sapling (3.5 m high) beside a fence-post. Magpies were nesting in a Camphor Laurel 70 m east of the original nest-tree. Composition of social group and the identity of individuals visiting the nest(s) In the first three seasons the breeding group consisted of five birds—two females, a male, and two helpers or auxiliaries (Table 1). In the fourth year, the group comprised a female, a male, and two helpers. Following the failed breeding attempt in 1997, a fifth bird (in adult plumage) associated with the group. In the 1994 and 1996 seasons, both helpers were immature (brown-plumaged) birds; in 1995, both helpers had adult plumage; and in 1997, one helper was immature and the other had near-adult plumage, separable from adults by its blackish-brown (not black) back and generally less neat appearance. No birds were marked, thus limiting data collection on the roles and contributions of members of the group. Immatures were distinguishable from others in the group, but not from each other. In years when there were two immature helpers, nest attendance by one could be monitored if both were simultaneously in view, and the second was inactive.